[RPG] n’t one make changes to concentration in homebrew

In 5E, the design standard seems be that almost all attacks - especially weapon attacks - feature a separate roll for each target. An “attack roll” in 5E is by convention a single attack vs a single target. Multiple target attacks generally have separate attack rolls made by the attacker (as in the case of multiattack actions), but if not, there's usually no attack roll and instead each target makes a saving throw to avoid some or all of the effect of the attack.

For melee attacks, there are two exceptions: the variant "Cleaving through Creatures" rules in the DMG and the Battle Master's Sweeping Attack manoeuvre, both of which allow an attacker to target a second creature as if making a second attack, but use the result of the initial attack roll. Both have limitations on when they can be used, though, and aside from reusing the initial roll, both are a second, separate attack. (Thanks to KorvinStarmast and Slagmoth for pointing these out.)

There are more examples of single attack vs multiple targets when it comes to spells: auto-hit spells like Magic Missile, which forgo a roll altogether (and as a result cannot critically hit), and unusual spells like Sleep or Color Spray which have the caster roll for a number of hit points of creatures who will be affected (but these spells don’t inflict damage).

An attack that hits more than one target with a single roll has several strengths and drawbacks that are not already present in the system:

• a single roll will be compared against multiple, potentially different armour classes - not gamebreaking, but it may remove some tactical options;
• it allows a creature to potentially score multiple critical hits with a single die roll - something not intended by the system;
• most importantly, there may be circumstances in which advantage or disadvantage applies against one or more targets, but not all - in which case, do you apply it to the roll or not? Rules for advantage and disadvantage may not allow for this case, and may become more or less powerful than intended depending on how you rule this.

I think these are all reasons why this was not a feature included in the game design, and so I’d be cautious about adding in your homebrew creations.

The Dungeon Master's Guide strongly recommends against this.

Dungeon Master's Guide, p.263:

Beware of adding anything to your game that allows a character to concentrate on more than one effect at a time, use more than one reaction or bonus action per round, or attune to more than three magic items at a time. Rules and game elements that override the rules for concentration, reactions, bonus actions, and magic item attunement can seriously unbalance or overcomplicate your game.

Here's why:

In earlier editions of the game, which had no one-spell concentration limit, characters could load up on buff spells before a fight or summon multiple creatures at once. Preparation rounds would give a significant advantage. It rewarded players who cast all their daily spells in one combat, leading to the "five minute adventuring day", where the party fully rests after every encounter.

It also overcomplicated the game because players often had to track multiple ongoing spell effects. It was too easy to forget your effects, leading to annoying situations where a player demands a do-over on three rounds of combat he forgot he was invisible, or a character who dies only to pop back to life when they remember a defensive spell that should have applied. I can't count the number of times someone in my D&D 3rd edition game forgot to apply an effect.

D&D 4th edition powers (i.e. attacks, spells and the like) often caused status effects, which were likewise easy to forget and a real hassle to track. D&D 5e's decision to limit each spellcaster to a single ongoing effect considerably reduces the number of possible "buffs" and "debuffs" on the battlefield at any one time. The DM spends less time adjudicating and tracking ongoing effects, which speeds gameplay. In my personal experience, combat could be very slow in D&D 4e and high-level D&D 3e, so the one-effect concentration limit is of significant importance.